An experimental drug has shown promise in extending the lives of women suffering from a particularly aggressive and deadly type of breast cancer, according to the results of a phase 2 trial.

Right now, the standard treatment of chemotherapy for metastatic triple-negative breast cancer has not been very effective. That might change with the new drug, called sacituzumab govitecan, which combines an antibody with a chemotherapy drug to better target cancer cells.

In the trial, funded by the drug's maker, about a third of the patients responded to treatment and the effect lasted up to eight months, said study senior author Dr. Kevin Kalinsky.

"These patients had a significant reduction of their cancer," he added. "This is an exciting new therapeutic."

About 50 percent had some response and saw a slowing of cancer progression for about five months, he noted.

Kalinsky, an oncologist at the Irving Medical Center at New York-Presbyterian-Columbia University, also noted: "This is a patient population where we really need new therapeutics."

Triple-negative breast cancer represents about 15 percent of all breast cancer and is most often seen in young women and black women, although why it affects these women is not known, according to Dr. J. Leonard Lichtenfeld, the interim chief medical officer at the American Cancer Society.

The cancer is different from other breast cancers because tumor cells do not have protein HER2 (human epidermal growth factor receptor 2), estrogen receptors or progesterone receptors, which means that hormonal treatment or drugs that target HER2 are ineffective. Chemotherapy shrinks or slows its progress in only about 10 percent to 15 percent of these patients, and the effect only lasts about two to three months, the researchers said.

Survival for women with metastatic triple-negative breast cancer is around a year, and that has not changed for 20 years, they noted.

In this phase 2 trial, 108 women who had failed treatment with other drugs were treated with sacituzumab govitecan. The drug combines an antibody to the Trop-2 antigen, which is found on most breast cancer cells, with the chemotherapy drug irinotecan.

When the antibody latches onto the Trop-2 molecules of the cancer cells, the drug is released within cells and into the areas of the tumor. This targeted delivery allows the chemotherapy drug to reach cancer cells and helps reduce the toxic effect of the drug to healthy cells, the researchers said.

The trial lasted from June 2013 to February 2017 and was funded by New Jersey-based Immunomedics.

Patients were given the drug for as long as they benefited from treatment, and eight women were still on the drug as of December 2017. The average time of treatment was more than five months, which was longer than the average 2.5 months with patients' other treatments, the researchers found.

Some of the drug's side effects included nausea, diarrhea, and a drop in white blood cells. Researchers said these symptoms were manageable.

The drug is not yet available, Kalinsky said. A phase 3 trial comparing sacituzumab govitecan with other treatments for triple-negative breast cancer is now underway.

"This is a step forward for women where there were not any real treatment options," said Lichtenfeld, who was not involved with the study. "It was not a cure – we still have a long way to go."

He added the drug might be more effective if it were given earlier, when the cancer was still confined to the breast.

"Even with these results, however, it represents a better outcome than any of the treatments used in this form of breast cancer," Lichtenfeld said.